Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 146–157

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A DFT study of the ionization and electron attachment of 2-azido pyridine Mohamed Elshakre ⇑ Chemistry Department, College of Science, Cairo University, University Avenue, Dokki 12613, Cairo, Egypt

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Stability sequence is cis-, tetrazole,

The ionization process from the S0 to the D0+ state (D0 S0+ transition) of cis-2 azido pyridine requires an energy of 8.76 eV to take an electron from the HOMO of the S0 state to infinity, which is accompanied by a subsequent extensive reduction of electron density of the HOMO after ionization.

and trans- in S0.  Stability sequence is cis-, trans-, and

tetrazole in D0+.  Stability sequence is trans-, cis-, and

tetrazole in D0.  Single point excited states of all

isomers resulting from p–p⁄ transitions.  Ionization and electron attachment result in changes of geometry and atomic charge.

a r t i c l e

i n f o

Article history: Received 14 May 2014 Received in revised form 18 October 2014 Accepted 23 October 2014 Available online 20 November 2014 Keywords: Excited states Sn Cationic state D0+ Anionic state D0 Vertical excitation energy Ionization energy Electron attachment energy

a b s t r a c t A DFT study using B3LYP/6-31G(d,p) level of theory is pursued to investigate the energy and geometry changes of 2-azido pyridine isomers in the S0, D+0, and D 0 . In S0, cis-2 azidopyridine is the most stable, followed by cyclic tetrazole and trans isomer is the least stable. In D+0, the cis isomer is the most stable, followed by trans isomer, and tetrazole isomer is the least stable. In the D 0 state, the trans form is the most stable, followed by the cis and the tetrazolo form is the least stable. Single point vertical excitation energy calculations of the cis form gave four excited states at 266.4, 239.4, 199.4, and 196.1 nm, all having p–p⁄ character. For trans isomer, five excited states at, 256.7, 250.3, 229.7, 199.0, 197.0 nm all resulting from p–p⁄ transitions. For the tetrazole isomer, seven excited states at 269.3, 242.7, 198.0, 187.2, 183.9, 182.1, 178.7 nm all resulting from p–p⁄ transitions. The geometries of the 3 isomers show noticeable changes in bond lengths, bond angles and dihedral angles, upon ionization and electron attachment. The ionization results in a remarkable variation of the NBO atomic charges as a result of ionization and electron attachment. where electron density of HOMO of the D+0 state is found to be much lower than that of the HOMO of the S0 in the case of ionization. The dipole moment calculations show that tetrazole isomer has the largest polarization in the S0 state, followed by cis isomer and trans isomer is the least polarized. The same pattern is found in the D+0 state. In the D 0 state, trans isomer is the highest polarized, followed by the cis, and tetrazole isomer is the least polarized. The rotational constants calculations of cis- isomer show that the cation is elongated along the long in-plane axis and compressed along the two short out-of-plane axes while the anion is compressed along the long in-plane axis, and elongated along the two short out-of plane axes. The cation and anion of trans isomer show similar behavior to the cis isomer. The cation of

⇑ Tel.: +20 2356 76702; fax: +20 2568 35799. E-mail addresses: [email protected], [email protected] http://dx.doi.org/10.1016/j.saa.2014.10.085 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

M. Elshakre / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 146–157

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cyclic tetrazole, is compressed along the long in-plane axis and elongated along the two short out-ofplane axes, while the anion elongates along the 3 mutually perpendicular axes. Ó 2014 Elsevier B.V. All rights reserved.

Introduction The photochemistry of azides of heterocyclic aromatics containing annular nitrogen is a rich area which constitutes various photochemical events. These azides can form cyclic five membered tetrazole ring upon the absorption of photons, when the azide group is adjacent to the annular nitrogen. This cyclic form containing the tetrazole ring can exist in equilibrium with the linear form, as shown in Fig. 1, for 2-azidopyridine [1–3]. On the other hand, when the azide group is not adjacent to the annular nitrogen atom, the absorption of photons can lead to its photolysis and the cleavage of the azide group producing various photo products. One of the common reported photodissociation pathways is the production of nitrene free radicals and molecular nitrogen as demonstrated by Budyka [4–6]. In contrast, recently, Morokuma et al. [7] and Wodtke et al. [8,9] have reported the formation of cyclic N3 upon the photolysis of azides other than heteroaromatics, e.g. Cl–N3 and CH3N3, respectively. Because of the floppy nature of the N3 group and its ability to rotate around the C–N bond, it can exist as cis or trans conformers, as shown in Fig. 1. The photostability of these molecules as drug and pharmaceutical candidates [10–16] requires an understanding of the energetics of the ground state cyclization process. Moreover, no information is available on the nature of the ionization forming the cationic state D+0 and electronic attachment forming the anionic state, (D0) state of 2-azidopyridine. It is the objective of this work to investigate the energies and geometries of these molecules in the S0, D+0 and D 0 states to examine the energetics and structural changes of 2-azidopyridine during ionization and electron attachment. The dynamics of cyclization and dissociation can be explored using femtosecond spectroscopic techniques [17], where femtosecond pump–probe photoelectron-photoion spectroscopy, which requires information of the ionization potential and electron attachment energies, can be applied to circumvent the complex dynamical processes of cyclization and dissociation of 2-azidopyridine. This requires the study of the ground, excited and ionic states of 2-azidopyridine. Therefore, I plan to investigate the electronic states of 2- azido-pyridines in its linear and cyclic forms using DFT, where the results from the excited state can provide information of the pump wave length needed to induce photochemistry and the results from the ionic state and the excited state can provide information of the probe wave length, used to detect the photochemical products by ionizing them. Budyka and his co-workers [4] studied the photo dissociations of 4-azido pyridine experimentally. The results of the quantum yields of the azido group photodissociation measured for the

protonated and methylated derivatives at the endocyclic nitrogen atom, showed a value of 0.49 for the dissociation yield of the neutral azide in MeCN which decreases to 0.22 for the positively charged derivatives. Budyka and Zyubina [6] performed calculations at the MNDO-PM3 and the ab initio level using UHF/6-31G⁄ base set for the neutral molecules S0 state, the lowest excited states (T0 and S1) and the radical anions (D0 and D1) of 4-azido pyridine. The results of their calculations showed that the decomposition of azido group is facilitated by the excitation of the azide into the lowest excited states and formation of the radical anion. Kanyalkar et al. [18] studied the substituent effect of the CH3, OH, Cl, OCH3, NO2 and COOH in position 5 with respect to the heterocyclic nitrogen of the pyridine ring in tetrazolo [1,5-a] pyridines, on the tetrazole-azide equilibrium, at the MP2/6-31G⁄⁄ level of theory, where their results showed that tetrazole forms are largely favored for the parent and 5-CH3 compounds, while the substituents including the OH, Cl, OCH3, and NO2-stabilize the azide isomer. Monajemi et al. [19] carried out calculations of the tetrazoleazides equilibrium of 2-azido pyridine and its substituents at the B3LYP/6-31++G⁄⁄ level of theory in order to investigate the nuclear quadropole resonance (NQR) parameters, including quadropole coupling constants, asymmetry factor, and NQR frequencies. Their results were found to be in agreement with those found by Kanyalakar et al. [18]. Abu Eittah et al. [20] investigated the height of the rotational barriers around the C (pyridine)–N (azide) single bond in 2, 3, 4 azidopyridines using ab initio molecular orbital calculations. They showed that the cis- conformer of 2-azidopyridine is slightly more stable than the trans- conformer. They showed that the height of the rotational barrier of the azide group in 2-azidopyridine is about 7 kcal/mol. Abu-Eittaha and his co-worker [21] studied the electronic absorption spectra of 2-azidopyridine in a wide variety of polar and non-polar solvents. They showed that the spectra of azido pyridines are different from those of pyridine and mono substituted pyridine. The spectra of the studied azido pyridines are characterized by the existence of overlapping, which was attributed to the p ? p⁄ transitions and the n ? p⁄ and may possibly be overlapped with the stronger p ? p⁄ transition.

Computational details The optimized geometry of the S0 state was calculated using the Gaussian 09 package [22]. The labeling of the atoms of all forms of 2-azido pyridine is shown in Fig. 2. The Density Functional Theory (DFT) with the B3LYP functional is used in this investigation. Geometry optimizations using valence triple zeta 6-311 + G (d, p) basis set, were performed for the S0 following the HF optimization of the S0 state. For the D+0 and D 0 state calculation, the UB3LYP

Fig. 1. The equilibrium between linear azide and cyclic tetrazole of 2-azido pyridine.

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M. Elshakre / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 146–157 Table 1 Total electronic energy au, and relative energy, kcal/mol of 2-azido pyridine isomers in the S0, D+0, and D 0 at B3LYP/6-311 + G(d,p). Total electronic energy

Relative energy

S0

transTS1 cisTS2 tetrazole

411.98504382 411.97881019 411.99174086 411.95935477 411.98904714

4.20 8.11 0.00 20.32 1.69

D+0

transTS1 cisTS2 tetrazole

411.66308302 411.63860146 411.67516787 411.64157745 411.65168175

7.58 22.95 0.00 21.08 14.74

D 0

transTS1 cisTS2 tetrazole

412.02298350 412.00174608 412.02205494 412.00979496 412.00516401

0.00 13.33 0.58 8.28 11.18

Table 2 Ionization energy and electron attachment energy eV, of 2-azido pyridine isomers at B3LYP/6-311 + G(d,p).

cisTS1 transTS2 tetrazole

Fig. 2. The isomeric forms of 2-azido pyridine (a) cis (b) TS1 (c) trans (d) TS2, and (e) cyclic tetrazole.

perturbation methods were performed using the 6-311 + G (d, p) basis set following the B3LYP optimization of the S0 state. Transition states were determined by varying the N1–N13 distance and optimizing the remaining structural parameters for each choice of the N1–N13 distance. The structure at the saddle point, the ‘‘calculate transition states’’ option was switched on to optimize the TS structure. Vibrational frequencies were calculated at the B3LYP/6-311 + G(d,p) level to confirm the nature of all stationary points. Time dependent density functional TD-DFT was used to calculate the single point vertical excitations. Results and discussion

Ionization energy

Electron attachment energy

8.76 9.25 8.61 8.64 9.18

1.03 0.62 0.82 1.37 0.43

trans- isomers, transition states TS1 and TS2 in the S0, D+0, D 0 states are given Table 1, which is expressed in atomic units (au) at B3LYP/ 6-311 + G (d, p) level of theory. The electronic energy was not corrected to the zero point vibrational energy. The results of the relative energy indicate that the cis isomer is the most stable, followed by the cyclic tetrazole, while the trans- form is the least stable. The energies of TS1 and TS2 indicate that the barrier involving rotation about C2–N11 is lower than that to cyclization forming the tetrazole ring by12.21 kcl/mol. In the D+0 state, the results indicate that the most stable isomer is the cis isomer as in the S0, followed by the trans isomer, while the terazole isomers is the least stable. The transition states TS1 and TS2 are located at 22.95 kcal/mol and 21.08 kcal/mol above the cis-form, respectively. The results in the D 0 state indicate that the most stable isomer is the trans form, followed by the cis form which is 0.58 kcal/mol higher than the trans form, while the tetrazole form is the least stable having 11.8 kcal/mol higher than the trans form. The transition states TS1 and TS2 are located at 13.33 and 8.28 kcl/mol, above the trans- form respectively.

Atom numbering and tautomer assignment

Ionization and electron attachment energies

The various isomers of 2-azido pyridine are shown in Fig. 2 indicating the atom numbering and labeling using CHEMDRAW [23] and CHEMCRAFT software [24] for dipole moments.

The journey of the linear molecule from linear azides to cyclic tetrazole passes through two transition states TS1 and TS2 can be probed using photoelectron and photoion spectroscopy, which requires the calculation of the ionization energy of all the species involved in the cyclization process including TS1 and TS2. The ionization energy is calculated as the energy difference between the optimized energy of the D+0 state and that of the S0 state. The results are shown in Table 2, which indicates that the ionization energy of the cis-form is 0.15 eV higher than that of the trans- form, while the ionization energy of the cyclic tetrazole is the highest; 0.57 eV higher than the trans- form and 0.42 eV higher than the cis-form. The ionization energies of the two transition states TS1 and TS2 indicate that the ionization energy of the transition state

Total electronic and relative (stability) energy In the linear configuration the N3 group can assume either a cis or trans configuration with respect to the pyridine ring. Passage from one linear form to another passes through a transition state, TS1 involving free rotation of the N3 about C2–N11 bond. Transformation from linear to cyclic form requires the passage through transition state, TS2 which involves cyclization. The total electronic and relative (stability) energies of the cis, cyclic terazole,

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M. Elshakre / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 146–157 Table 3 Calculated single point vertical excitations of cis-2-azidopyridine using TD-B3LYP/6-311+G(d,p) level of theory. Configuration

Orbitals involved

Coefficient

E (eV)

k (nm)

f

Assignment

S1

29->34 31->32 31->34

H-2 ? L + 2 H?L H?L+2

0.146 0.639 0.229

4.6

266.4

0.134

p–p⁄

S2

27->33 29->32 31->32 31->34

H-4 ? L + 1 H-2 ? L H?L H?L+2

0.172 0.173 0.226 0.615

5.2

239.4

0.235

p–p⁄

S3

27->33 28->32 28->34 29->32 29->34 31->34 31->36

H-4 ? L + 1 H-3 ? L H-3 ? L + 2 H-2 ? L H-2 ? L + 2 H?L+2 H?L+4

0.305 0.287 0.129 0.244 0.306 0.188 0.336

6.2

199.4

0.049

p–p⁄

S4

27->33 28->32 28->34 29->32 31->36

H-4 ? L + 1 H-3 ? L H-3 ? L + 2 H-2 ? L H?L+4

0.188 0.138 0.137 0.493 0.405

6.3

196.1

0.020

p–p⁄

involving rotation around C2–N11 bond is higher than the transition state involving cyclization to form tetrazole by 0.61 eV. The electron attachment energy is calculated as the difference between the optimized energy of the D 0 and S0 states, as shown in Table 2, which indicates that the highest electron attachment energy is that of the cis-form and the lowest is that of the cyclic tetrazole.

Vertical excitation energies For each isomer, several excited states are computed and the transitions involved are assigned, where each transition resulting from several excitations between various molecular orbitals, which are given by the general form

Ψ29, HOMO-2

Ψ32, LUMO

Ψ31, HOMO

Ψ34, LUMO+2

Ψ36, LUMO+4 Fig. 3. Molecular orbitals of the 4 excited states of cis-2azido pyridine, W29 HOMO-2, W31 HOMO, W32 LUMO, W34 LUMO + 2, and W36LUMO + 4.

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M. Elshakre / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 146–157

Table 4 Calculated single point vertical excitation of trans-2-azidopyridine using TD-B3LYP/6-311 + G(d,p) level of theory. Configuration

Orbitals involved

Coefficient

E (eV)

k (nm)

f

Assignment

S1

29->32 31->32 31->34

H-2 ? L H?L H?L+2

0.218 0.238 0.619

4.8

256.7

0.065

p–p⁄

S2

28->33 30->33 31->32 31->34

H-3 ? L + 1 H-1 ? L + 1 H?L H?L+2

0.189 0.234 0.583 0.232

4.9

250.3

0.204

p–p⁄

S3

30->33 31->32

H-1 ? L + 1 H?L

0.663 0.212

5.4

229.7

0.056

p–p⁄

S4

27->32 27->34 28->33 29->32 29->34 31->34 31->36

H-4 ? L H-4 ? L + 2 H-3 ? L + 1 H-2 ? L H-2 ? L + 2 H?L+2 H?L+4

0.188 0.183 0.335 0.411 0.200 0.176 0.249

6.2

199

0.036

p–p⁄

S5

27->34 28->33 29->32 29->34 31->32 31->36

H-4 ? L + 2 H-3 ? L + 1 H-2 ? L H-2 ? L + 2 H?L H?L+4

0.186 0.220 0.424 0.184 0.108 0.418

6.3

197

0.060

p–p⁄

Wij ¼ Rcij /ij

ð1Þ

where Wij is the molecular orbital and /ij’s are the atomic orbitals contributing to the formation of the molecular orbital. For cis- isomer, four singlet excited states are calculated as shown in Table 3, which are designated as S1, S2, S3, and S4 respectively. Excited state S1 has energy of 4.6 eV above S0, corresponding to a wavelength of 266.4 nm and an oscillator strength of 0.134. For this state, the calculations show that there are five molecular orbitals involved. The transition with the largest coefficient 0.639 corresponds to a transition from orbital W31, a HOMO to orbital W32 a LUMO, which is assigned as p–p⁄ transition. S2 is located 5 eV above S0 in the Frank–Condon region of the potential energy surface (PES), corresponding to a

Ψ29, HOMO-2

Ψ32, LUMO

wavelength of 239.4 nm having an oscillator strength of 0.235 showing that it is the strongest transition with the highest oscillator strengths. For this excited state, the transition with the largest coefficient of 0.615 corresponds to a transition from a HOMO to a LUMO + 2, which is a p–p⁄. S3 is located 6.2 eV above S0 corresponding to a wavelength of 199.4 nm with smaller oscillator strength of 0.049. The transition with the largest coefficient of 0.336 corresponds to a transition from W31 a HOMO to W36, a L + 4 which have p and p⁄ character. S4 is found to be 6.3 eV above S0 corresponding to a wavelength of 196.1 nm, having the lowest oscillator strength of 0.02, which results from the transition between the two orbitals H-2 and L, having p and p⁄ character. The molecular orbitals involved in the four transitions are shown in Fig. 3.

Ψ31, HOMO

Ψ34, LUMO+2

Fig. 4. Molecular orbitals of the 3 selected excited states of trans-2azido pyridine, W29 HOMO-2, W31 HOMO, W32 LUMO, and W34 LUMO + 2.

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M. Elshakre / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 146–157 Table 5 Calculated single point vertical excitation of tetrazolo-2-azidopyridine using TD-B3LYP/6-311 + G(d,p) level of theory. Configuration

Orbitals involved

Coefficient

E (eV)

k (nm)

f

Type

S1

29->33 31->32

H-2 ? L + 1 H?L

0.102 0.683

4.6

269.3

0.061

n–p⁄

S2

29->32 31->33

H-2 ? L H?L+1

0.440 0.540

5.1

242.7

0.023

p–p⁄

S3

29->32 29->33 31->33 31->35

H-2 ? L H-2 ? L + 1 H?L+1 H?L+3

0.459 0.142 0.385 0.308

6.3

198.0

0.415

p–p⁄

S4

27->32 29->32 29->33 31->33 31->35

H-4 ? L H-2 ? L H-2 ? L + 1 H?L+1 H?L+3

0.119 0.237 0.465 0.193 0.385

6.6

187.2

0.191

p–p⁄

S5

31->36

H?L+4

0.701

6.7

183.9

0.012

p–r⁄

S6

26->32 30->35

H-5 ? L H-1 ? L + 3

0.121 0.678

6.8

182.1

0.012

n–p⁄

S7

27->32 29->33 30->34 31->35

H-4 ? L H-2 ? L + 1 H-1 ? L + 2 H?L+3

0.302 0.381 0.169 0.439

6.9

178.7

0.083

p–p⁄

For trans- isomer, five excited electronic states, S1, S2, S3, S4 and S5 are computed, as shown in Table 4. Three excited states; S1, S2, and S3 are selected. S1 is found at 4.8 eV above S0, having an oscillator strengths of 0.065 corresponding to a wavelength of 256.7 nm. The molecular orbitals having the largest coefficient of 0.619 are W31, a HOMO and W34, LUMO + 2 having p and p⁄ characters, respectively. S2 is found at 4.9 eV above S0 corresponding to a wavelength of 250.3 nm, with oscillator strength of 0.204. The transition with the largest coefficient 0.583 is that involving orbitals W31, a HOMO having p character and W32, a LUMO having p⁄ character. S4 is found at 6.2 eV above S0 with wavelength of 199 nm with oscillator strength of 0.036. The molecular orbitals W29 and W32 involved in this transition, having a coefficient of 0.441, where W29 is a HOMO-2 with p character and W32 is a LUMO with p⁄ character. The molecular orbitals involved in the transitions are shown in Fig. 4. There are 7 excited states calculated for cyclic tetrazolo pyridine, as shown in Table 5. 4 excited states; S1, S2, S3, and S4 are considered. S1 is found at 4.6 eV above S0, corresponding to a

Ψ29, HOMO-2

Ψ32, LUMO

wavelength of 269.3 nm with oscillator strength of 0.061, where the major contribution to this state comes from W31 and W32 with the largest coefficient of 0.683. W31 is a HOMO having p character, while W32 is a LUMO having p⁄ character. S2 is found 5.1 eV above S0 at wavelength of 242.7 nm with oscillator strength of 0.023, which involves W31 and W33, where W33 is a LUMO + 1 having p⁄ character. S3 is located at 6.3 eV above S0 at 198 nm. The molecular orbitals involved in this transition are W29 and W32, which are assigned as HOMO-2 and LUMO having p and p⁄ character, respectively. S4 is located 6.6 eV above S0, corresponding to 187.2 nm, where W29 and W33 involved in this transition with W29 is HOMO-2 with p character and W33 is LUMO + 1with p⁄ character. The molecular orbitals involved in the transitions are shown in Fig. 5. The results of the theoretical vertical excitation energy of the cis-, trans-, and cyclic tetrazole are shown in Fig. 6a, and compared with the experimental UV spectra in absolute ethanol of Ref. [21], shown in Fig. 6b. The experimental UV spectrum of 2-azido pyridine shows three bands, one strong band at 216 nm, a second at 244 nm, and a

Ψ31, HOMO

Ψ33, LUMO+1

Fig. 5. Molecular orbitals of the 4 excited states of tetrazolo-2azido pyridine, W29 HOMO-2, W31 HOMO, W32 LUMO, and W33 LUMO + 1.

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M. Elshakre / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 146–157

Fig. 6. (a) Theoretical UV spectra of cis, trans, and tetrazole isomers of 2-azidopyridine calculated at B3LYP/6-31G(d,p) level of theory, and (b) experimental UV spectrum of 2azidopyridine in absolute ethanol [adapted from Ref. [21].

third with vibrational structure at 265 nm. The theoretical UV spectrum of cis isomer gives two strong transitions at 266 and 239 nm, while the trans-form gives three strong transitions at 256, 250, and 229 nm. The spectrum of the tetrazole isomer gives three strong transitions at 269, 242, and 198 nm. The comparison of the experimental and theoretical UV spectra shows that the experimental bands at 265 and 244 nm are reproduced by the theoretical UV spectra of the tetrazole isomer at 269 nm and 242 nm, respectively, while the strong experimental band at 216 nm is reproduced by the theoretical band at 229 nm of the trans isomer, indicating a good agreement between experiment and theory.

Molecular structure Bond length, bond angles, and dihedral angles for each isomer in the S0, D+0, and D 0 states are compared to indicate the effect of ionization and electron attachment on the geometry and molecular structure. The results of the geometry of-2-azido pyridine in the three states are given in Table 6, where comparison of selected geometrical parameters of the various isomers and transition states in the three electronic states is made. For cis 2- azido pyridine, N1–C2 bond which is a double bond, elongates upon ionization and further elongates upon electron

Table 6 Selected geometrical parameters optimized at B3LYP/6-311 + G(d,p) level for the various isomers and transition states of 2-azidopyridine. D+0

S0 cisBond lengths (Å) r(N1–C2) r(N1–C6) r(C2–C3) r(C2–N11) r(C3–C4) r(C4–C5) r(C5–C6) r(N11–N12) r(N12–N13) r(N13–N1) Bond Angles (°) \ (C2–N1–C6) \ (N1–C2–C3) \ (N1–C2–N11) \ (C3–C2–N11) \ (C2–C3–C4) \ (C3–C4–C5) \ (C4–C5–C6) \ (N1–C6–C5) \ (C2–N11–N12) \ (N11–N12–N13) \ (N1–N13–N12) \ (C2–N1–N13)

1.330 1.335 1.403 1.419 1.388 1.394 1.392 1.233 1.133

TS1 1.326 1.338 1.397 1.440 1.390 1.393 1.392 1.230 1.133

trans1.331 1.338 1.401 1.413 1.386 1.397 1.391 1.239 1.128 3.356

TS2 1.328 1.341 1.409 1.396 1.379 1.408 1.382 1.305 1.173 2.002

D 0

tetrazole cis1.375 1.368 1.414 1.333 1.370 1.427 1.363 1.342 1.299 1.360

1.354 1.310 1.435 1.362 1.380 1.398 1.431 1.266 1.120

TS1 1.346 1.313 1.437 1.360 1.382 1.395 1.435 1.219 1.127

trans1.354 1.312 1.431 1.356 1.383 1.395 1.432 1.277 1.115 3.372

TS2 1.345 1.315 1.418 1.352 1.394 1.393 1.420 1.381 1.167 1.917

tetrazole cis 1.366 1.339 1.400 1.344 1.407 1.394 1.404 1.375 1.268 1.418

1.367 1.329 1.436 1.359 1.378 1.403 1.394 1.378 1.189

TS1 1.359 1.335 1.421 1.380 1.384 1.399 1.393 1.347 1.196

trans1.365 1.332 1.436 1.353 1.375 1.408 1.392 1.383 1.186 3.855

TS2 1.360 1.335 1.439 1.357 1.373 1.410 1.389 1.387 1.196 2.717

tetrazole 1.374 1.409 1.412 1.347 1.430 1.392 1.402 1.356 1.303 1.352

117.63 123.53 113.43 123.04 117.89 119.26 117.91 123.77 118.26 172.45

117.46 124.02 116.35 119.51 117.87 118.93 118.25 123.47 117.56 172.58

117.59 123.85 119.24 116.91 117.66 119.34 118.09 123.47 116.57 172.57 40.81 75.96

122.10 120.93 116.90 122.17 117.68 120.37 118.52 120.40 104.68 132.97 90.17 95.28

124.09 118.82 107.56 133.62 118.00 121.06 120.50 117.53 105.89 112.94 105.43 108.18

117.25 123.82 112.10 124.08 117.97 118.36 119.21 123.39 119.68 168.41

117.14 123.92 116.87 119.06 118.07 118.02 119.38 123.27 138.15 173.85

116.88 124.43 119.56 116.01 117.63 118.33 119.48 123.25 116.32 169.37 38.31 75.18

122.87 121.00 116.65 122.35 117.32 119.95 119.64 119.22 105.57 126.32 95.04 96.42

125.03 119.44 108.13 132.43 117.28 120.89 120.31 117.05 105.77 113.11 105.72 107.28

118.90 119.30 114.82 125.88 119.94 120.11 116.31 125.44 111.83 127.20

118.69 119.72 119.88 120.22 120.61 118.93 117.13 124.90 115.63 129.69

118.46 119.53 124.68 115.79 120.23 119.68 116.45 125.65 113.27 126.21 6.79 61.47

118.68 119.47 125.46 115.07 120.27 119.54 116.61 125.42 118.61 131.97 82.27 81.69

124.21 120.00 106.71 133.29 117.08 121.02 122.06 115.63 106.32 111.79 106.19 109.00

Dihedral angle (°) D(N1–C2–C3–C4) 0.00 D(N1–C2–N11–N12) 180.00 D(C3–C2–N11–N12) 0.01 D(C2–N11–N12–N13) 179.46 D(N11–N12–N13–N1) 179.46

0.93 101.11 82.66 179.68 139.29

0.00 0.01 179.99 176.98 179.98

0.00 0.00 180.00 0.00 0.00

0.00 0.00 179.99 0.03 0.04

0.00 180.00 0.01 179.85 179.85

5.25 97.16 87.05 177.26 132.51

0.00 0.00 180.00 180.00 180.00

0.00 0.00 180.00 0.00 0.00

0.00 0.01 180.00 0.02 0.03

0.27 178.30 2.17 177.79 0.47

1.08 93.48 91.39 179.05 62.60

0.02 0.04 179.97 179.99 179.99

0.04 0.02 179.98 0.05 0.04

0.00 0.01 179.99 0.03 0.03

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M. Elshakre / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 146–157 Table 7 Selected NBO charge at B3LYP/6-311 + G (d,p) level for the various isomers of 2-azidopyridine. D+0

S0

N1 C2 C3 C4 C5 C6 H7 H8 H9 H10 N11 N12 N13

D 0

cis-

TS1

trans-

TS2

tetrazole

cis-

TS1

trans-

TS2

tetrazole

cis-

TS1

trans-

TS2

tetrazole

0.448 0.349 0.276 0.152 0.259 0.067 0.218 0.214 0.217 0.193 0.316 0.236 0.042

0.448 0.352 0.248 0.157 0.247 0.062 0.224 0.215 0.218 0.194 0.348 0.242 0.060

0.497 0.353 0.245 0.155 0.263 0.068 0.227 0.213 0.217 0.193 0.358 0.258 0.011

0.419 0.351 0.223 0.153 0.261 0.076 0.232 0.217 0.222 0.209 0.386 0.102 0.034

0.213 0.318 0.208 0.177 0.241 0.051 0.238 0.220 0.227 0.232 0.309 0.041 0.095

0.392 0.398 0.148 0.170 0.060 0.131 0.242 0.255 0.247 0.231 0.172 0.224 0.215

0.401 0.423 0.097 0.182 0.054 0.145 0.253 0.258 0.248 0.232 0.210 0.249 0.137

0.458 0.394 0.098 0.175 0.078 0.145 0.259 0.256 0.247 0.230 0.208 0.245 0.242

0.355 0.311 0.073 0.147 0.124 0.166 0.267 0.260 0.258 0.253 0.137 0.102 0.219

0.232 0.282 0.055 0.106 0.166 0.200 0.274 0.262 0.268 0.270 0.094 0.022 0.075

0.516 0.362 0.309 0.187 0.363 0.050 0.212 0.182 0.187 0.158 0.477 0.079 0.221

0.496 0.391 0.263 0.195 0.315 0.042 0.204 0.186 0.190 0.163 0.551 0.060 0.296

0.540 0.365 0.275 0.196 0.366 0.056 0.203 0.183 0.187 0.158 0.513 0.035 0.227

0.530 0.356 0.258 0.205 0.357 0.053 0.203 0.182 0.187 0.159 0.525 0.063 0.201

0.251 0.331 0.414 0.292 0.284 0.175 0.199 0.181 0.187 0.195 0.401 0.131 0.145

attachment. The elongation of the bond upon ionization is attributed to the removal of an electron from HOMO, which alters the electronic distribution, decreasing the bonding character. Similarly, attaching an electron to the LUMO causes this bond to

elongate further. In TS1, ionization and electron attachment causes elongation of N1–C2. In trans 2-azido pyridine, ionization results in an elongation of the bond, while electron attachment results in further elongation. For TS2, ionization results in elongation of the

Fig. 7. HOMO of cis-2 azido pyridine (a) S0, and (b) D+0.

Fig. 8. HOMO of trans-2 azido pyridine (a) S0, and (b) D+0.

Fig. 9. HOMO of tetrazole -2 azido pyridine (a) S0, and (b) D+0.

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Table 8 Dipole moments (Debye, D) of the various isomers of 2-azidopyridine calculated at B3LYP/6-311 + G(d,p). S0

cisTS1 transTS2 tetrazole

3.42 3.93 3.16 4.26 6.47

D+0

cisTS1 transTS2 tetrazole

3.73 4.37 2.91 4.57 6.99

D 0

cisTS1 transTS2 tetrazole

7.15 9.41 7.32 6.72 3.96

N1–C2 bond length, while electron attachment results in further elongation. For cyclic tetrazole, ionization causes a noticeable reduction of the N1–C2 bond length, while electron attachment does not result in a change of the bond length. The bond angle C2–N1–C6 changes from 117.6 to 117.2 and 118.9 upon ionization and electron attachment, respectively for the cis isomer, which implies that ionization causes this angle to shrink, while electron attachment opens it up. The removal or addition of an electron from HOMO or LUMO leads to changing the electron densities resulting in a change of the bond angles. For TS1, the same bond angle slightly changes upon ionization and noticeably changes upon electron attachment. For trans-form, the same bond angle does not change upon ionization and slightly opens up upon electron attachment. For TS2, C2–N1–C6 angle slightly changes upon ionization and noticeably shrinks in D 0 . In cyclic tetrazole, C2–N1–C6 is 124.0, which changes to 125.0 and 124.2 in both D+0 and D 0 , respectively. For dihedral angles, the dihedral angle N1–C2–C3–C4 of cis-form does not change upon ionization while it changes by 0.27 upon electron attachment. For TS1, the same dihedral angle changes from 0.93 in the S0 to 5.25 upon ionization and 1.08 upon electron attachment, respectively. For the trans- form, the same dihedral angle does not change upon ionization and changes by 0.02 upon electron attachment. For TS2, this dihedral angle does not change upon ionization, while changing only by 0.04 upon electron attachment. For cyclic tetrazole isomer, this dihedral angle does not change upon ionization and electron attachment. The rest of the bond lengths, bond angles, and dihedral angles of the various isomers in the various electronic states can be analyzed in a similar fashion, as shown in Table 6. Atomic charges

Fig. 10. Dipole moment vector of cis- 2-azidopyridine in S0 (a) D+0, (b) and D 0 (c) states.

The natural bond orbital (NBO) charges of the atoms of the various isomers of 2-azidopyridine at B3LYP/6-311 + G(d,p) level of theory are shown in Table 7 for S0, D+0, D 0 states. NBO charges on N1 for cis-isomer in the S0 and D+0 are 0.448, 0.392, respectively, indicating that the ionization decreases the charge by 0.056. In the D 0 , the charge becomes 0.516 indicating that attachment of an electron to a LUMO increases the negative charge by 0.068. NBO charges on C2 in the S0, D+0, and D 0 are 0.349, 0.398, 0.362, indicating that ionization increase the positive charge by 0.049, while electron attachment to a LUMO increases the positive charge by 0.013. In S0 the NBO charge on C3 in S0, D+0, and D 0 is 0.276, 0.148, and 0.309, respectively, indicating that ionization reduces the negative charge by 0.128, while electron attachment of an electron to LUMO increases the negative charge by 0.033. For C4, NBO in the S0, D+0, and D 0 are 0.152, 0.170, and 0.187

indicating that ionization increases the negative charge by 0.018, while electron attachment increases the negative charge by 0.035. Similar analysis of the changes of atomic charges in the various electronic states can be followed using Table 7. The largest reduction of atomic charge as a result of ionization due to D+0 S0 transition is found to be on N13, indicating that ionization takes place from HOMO orbital involving atom N13, which is verified by comparing HOMO involving atom N13 in S0 and D+0, where the electron density is reduced quite extensively due to ionization, as shown in Fig. 7. For the trans-form, NBO charge on N1 in the S0, D+0, and D 0 are 0.497, 0.458 and 0.540, indicating that ionization lowers the negative charge and electron attachment increases the negative charge. NBO charge on C2 is 0.353, 0.394, and 0.365 in the S0, D+0,

M. Elshakre / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 146–157

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Fig. 11. Dipole moment vector of trans- 2-azidopyridine in S0 (a) D+0 (b) and D 0 (c) states.

and D 0 , respectively, indicating that ionization and electron attachment increase the positive charge. For C3, charge is 0.245, 0.098, and 0.275 in the S0, D+0, and D 0 , respectively, showing that ionization reduces the negative while electron attachment increases the negative charge. For C4, charge is 0.155, 0.175, and 0.196 in the S0, D+0, D 0 , respectively, implying that ionization and electron attachment increases the negative charge. Similar analysis of changes of atomic charges in the various electronic states can be achieved using Table 7. The largest reduction of atomic charge as a result of ionization due to D+0 S0 transition is found to be on N13 which indicates that ionization takes place from HOMO orbital involving atom N13, which is verified by

Fig. 12. Dipole moment vector of tetrazolo 2-azidopyridine isomer in S0 (a) D+0 (b) and D 0 (c) states.

comparing HOMO involving N13 in S0 and D+0, where the electron density is reduced quite extensively in the D+0 due to ionization compared with that in the S0, as shown in Fig. 8.

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Table 9 Rotational constants, GHz of 2-azidopyridine isomers in the S0, D+0, and D 0 at B3LYP/6-311+G(d,p). a

trans-

TS1

cis-

TS2

tetrazole

S0

A B C

5.016 1.120 0.915

4.698 1.086 0.953

5.017 1.141 0.930

4.263 1.530 1.126

4.146 1.761 1.236

D+0

A+ B+ C+ DA+ DB+ DC+

4.928 1.118 0.911 0.088 0.002 0.004

5.035 1.023 0.884 0.337 0.063 0.069

4.961 1.149 0.933 0.057 0.007 0.003

4.217 1.556 1.137 0.045 0.026 0.011

4.167 1.734 1.224 0.021 0.027 0.012

D 0

A B C DA DB DC

5.267 1.062 0.884 0.251 0.058 0.032

5.191 1.021 0.889 0.493 0.066 0.064

5.372 1.051 0.879 0.355 0.090 0.051

4.482 1.255 0.980 0.220 0.276 0.146

4.107 1.722 1.213 0.039 0.039 0.022

A+, B+, and C+ are the rotational constants around the 3 mutually perpendicular axes in the D+0 state. A, B, and C are the rotational constants around the 3 mutually perpendicular axes in the D 0 state. DA+ = A+D0  AS0, DB+ = B+D0  BS0, DC+ = C+D0  CS0.  + +    DA = AD0  AS0, DB = BD0  BS0, DC = CD0  CS0. a A, B, and C are the rotational constants around the 3 mutually perpendicular axes in the S0 state.

For the cyclic tetrazole form, the NBO charge on N1in the S0, D+0, and D 0 is 0.213, 0.232, and 0.251, respectively. NBO charge on C2 is 0.318, 0.282, and 0.331 in the S0, D+0, and D 0 , respectively. NBO on C3 is 0.208, 0.055, and 0.414 in the S0, D+0, and D 0 , respectively. Charge on C4 is 0.177, 0.106, and 0.292 in the S0, D+0, D 0, respectively. Similar analysis can be carried out using Table 7. The largest reduction of atomic charges as a result of ionization due to D+0 S+0 transition is found to be on N13, which indicates that ionization takes place from HOMO orbital involving atom N13, which is verified by comparing HOMO involving N13 in S0 and D+0, where the electron density is reduced quite extensively in the D+0 due to ionization compared with that in the S0, as shown in Fig. 9. Dipole moment The dipole moment measures the magnitude of polarization of the molecule. The dipole moment of a neutral molecule that has no net charge is independent of the origin of the center of mass of the molecule. For charged molecules, the dipole moment depends on the origin of the charged molecule, Hence the results of the dipole moments can be less reliable. However, if the dipole moment for the origin of the neutral molecule is given, one can calculate the dipole moment for the origin of the charged molecule by adding the total charge times the displacement vector from the old origin of the neutral to the new origin of the charged molecule. This attempt is not tried in this study and the results of the dipole moment of the molecules in the D+0 and D 0 is presented the way it is in the output file, without correction of the origin of the charged molecules. The values of the dipole moments and dipole moment vectors using ChemCraft [24] of the various isomers of 2-azidopyridine in the S0, D+0, and D 0 , are given in Table 8 and Figs. 10–12. The dipole moment of the cis-isomer in the S0, D+0, and D 0 is 3.42, 3.73, and 7.15D, respectively with the dipole moment vectors shown in Fig. 10a–c, respectively, indicating that ionization causes a slight increase of the polarization of cis 2-azidopyridine, while electron attachment results in a significant change. For the trans- isomer, the dipole moment in the S0, D+0, and D 0 state is 3.16, 2.91, and 7.32D, respectively, where the dipole moment vectors are shown in Fig. 11a–c, respectively, indicating that the molecular cation is less polarized than the neutral, while the molecular anion is significantly more polarized than the neutral. For cyclic tetrazole isomer, the dipole moment in the S0, D+0, and

D 0 state is 6.47, 6.99, and 3.96 D, respectively, with the dipole moment vectors given in Fig. 12a–c, respectively, indicating that cyclic cationic state is more polarized than cyclic neutral, while anionic state is the least polarized. Rotational constants Spectroscopically, the values of the rotational constants can give useful information about the structure and geometry of the various isomers of 2-azido pyridine in the various electronic states. The calculated rotational constants are shown in Table 9. It is known that any molecule possesses three rotational constants A, B and C which are related to three moments of intertia Ia, Ib, and Ic corresponding to rotation about the three perpendicular axes passing through the center of mass of the molecule [25], from which bond lengths and bond angles can be deduced. For cis-isomer, the rotational constants A, B, and C in S0 state are 5.01, 1.14, and 0.93 GHz, respectively. Ionization results in a change of the three rotational constants becoming 4.96, 1.14, and 0.93 GHz, respectively. The difference between the rotational constant in the D+0 and S0, DA+, DB+, and DC+ is 0.057, 0.007, and 0.003, respectively, indicating that ionization causing elongation along the long in-plane axis and compression along the two short out-of-plane axes. The rotational constants of the anion are 5.37, 1.05, and 0.87, respectively, where the difference between the    rotational constants in the D 0 and S0 states, DA , DB , and DC is 0.35, 0.09, and 0.05 indicating that the anion is compressed along the long in-plane axis, and elongated along the two short out-of plane axes. For trans-isomer, the rotational constants A, B, and C in the S0 state are 5.01, 1.12, and 0.91 GHz, respectively. The ionization results in a change of the three rotational constants becoming 4.92, 1.11, and 0.91 GHz, respectively, giving DA+, DB+, and DC+ values of 0.08, 0.002, and 0.003, respectively, indicating that ionization causing elongation along the long in-plane axis and one of the out-of-plane axes, while the molecule is compressed along the other out-of-plane axis. The values of the rotational constants of the anion are 5.26, 1.06, and 0.88, respectively, giving values of 0.25, 0.05, and 0.03 for DA, DB, and DC indicating that the anion is compressed along the long in-plane axis, and elongated along the two short out-of plane axes. For cyclic tetrazole isomer, the rotational constants A, B, and C in the S0 state are 4.14, 1.76, and 1.23 GHz, respectively, where

M. Elshakre / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 146–157

ionization results in a change of the rotational constants to become 4.16, 1.73, and 1.22 GHz, respectively. The difference DA+, DB+, and DC+ is 0.021, 0.027, and 0.012 shows that ionization causing compression along the long in-plane axis and elongation along the two short out-of-plane axes. In the D 0 , the constants are 4.10, 1.72, and 1.21, where the difference DA, DB, and DC is 0.03, 0.03, and 0.02 indicates that the anion is elongated along the three perpendicular axes. Conclusion B3LYP/6-31G(d,p) calculations were carried out to plore the energy and geometry changes of the various isomers of the linear 2-azido pyridine and the tetrazolo pyridine in S0, D+0, and D 0 . In S0 state, cis-2 azidopyridine is found to be the most stable isomer, cyclic tetrazole is 1.69 kcal/mol higher than cis form and the trans isomer is found to be the least stable with 4.2 kcal/mol higher than cis form. In the D+0 state, cis isomer is found to be the most stable as in S0, followed by the trans isomer with a stability energy of 7.58 kcal/mol, and tetrazole isomer is found to be the least stable with a stability energy of 14.74 kcal/mol. In D 0 state, the trans form is found to be the most stable, where cis form is 0.58 kcal/mol higher, and tetrazolo form is the least stable which is 11.18 kcal/ mol higher than the trans form. Single point vertical excitation energy calculations of the cis form gave four excited states; S1, S2, S3, and S4 located at 266. 4, 239.4, 199.4, and 196.1 nm, which all involve p–p⁄. For the trans isomer, five excited states; S1, S2, S3, S4, and S5 located at, 256.7, 250.3, 229.7, 199, 197 nm where all involve p–p⁄ transition. For the tetrazole isomer, seven excited states are calculated, S1, S2, S3, S4, S5, S6, and S7 located at 269.3, 242.7, 198.0, 187.2, 183.9, 182.1, 178.7 nm, where all states involve p–p⁄ transition. The geometries of the 3 isomers show noticeable changes upon the ionization and electron attachment, including variation of bond lengths, bond angles and dihedral angles. Ionization results in a remarkable variation of the NBO atomic charges as a result of ionization and electron attachment. The dipole moment calculations show that tetrazole isomer has the largest polarization in the S0 state with a D.M. of 6.47D, followed by cis isomer with a D.M. of 3.42D and trans isomer is the least polarized having a D.M. of 3.16D. The rotational constant calculations of the various isomers show that ionization and electron attachment cause noticeable changes of the three perpendicular moments of inertia. References [1] F.R. Benson, L.W. Hartzel, E.A. Otten, Azido pyrimidines, J. Am. Chem. Soc. 76 (1954) 1858–1862. [2] C. Temple Jr., J. Montgomery, Studies on the azidoazomethine-tetrazole equilibrium. I. 2-Azidopyrimidines, J. Org. Chem. 30 (1965) 826–829. [3] C. Temple Jr., R.L. McKee, J.A. Montgomery, studies on the azidoazomethinetetrazole equilibrium. II. 4-azidopyrimidines, J. Org. Chem. 30 (1965) 829–834. [4] F. Budyka, N.V. Biktimirova, T.N. Gavrishova, O.D. Laukhina, D.B. Zemtsov, Photodissociation of 4-azidopyridine and 4-azidoquinoline in neutral and cationic forms, J. Photochem. Photobiol. A: Chemistry 173 (2005) 70–74.

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